The expression well string is used to refer to any string or well tubular used for operations in a borehole, such a drilling, logging, fluid transportation. The well string does not necessarily need to be tubular over its entire length. The well string can in particular be a drill string.
In the course of an operation such as drilling into an earth formation, it is sometimes desired to have access to the borehole exterior of the well string. For example, performing measurements of certain properties of the surrounding earth formation is difficult if not impossible to be done from inside a drill string. Other examples in which access to the borehole is desired include obtaining a sample of the surrounding formation, injecting a fluid such as cement or lost circulation material for prevention of fluid losses, or performing a cleaning operation such as the removal of mudcake from the borehole wall e.g. by jet cleaning.
If it is known at the start of the drilling operation what operations need to be performed, it is sometimes possible to include specialised equipment in the well string, such as a dedicated Measurement-While-Drilling (MWD) tool. Such specialised equipment is expensive, and often the need for specialised equipment is only encountered in the course of the drilling operation. In such cases the drill string has to be pulled up to surface so that a special tool can be mounted before the drill string is run into the borehole again.
International patent application with publication No. WO 00/17488 discloses a system for drilling and logging of a wellbore formed in an earth formation, wherein a logging tool can be lowered in the wellbore from inside a tubular drill string through a drill bit at the lower end of the drill string.
The drill bit includes a bit body provided with a passageway for the logging tool, and a closure element for the passageway in the form of an insert section at the bit face. The bit body is attached to the drill string at a drill-string side of the bit body, and the passageway extends from an opening at the drill-string side to the well exterior of the bit body. The closure element comprises a primary latching device for selectively connecting the closure element to the bit body, so as to selectively close the passageway.
The known system further comprises an auxiliary tool for manipulating the closure element, which auxiliary tool forms the lower part of a logging tool string.
The logging tool string of the known system is arranged so that it can pass from the attached well string through the opening of the bit body at the drill-string side, along the passageway so that it can reach the closure element, when the closure element is connected to the bit body. The auxiliary tool comprises a secondary latching device for selectively connecting the auxiliary tool to the closure element. The secondary latching device is further so arranged that simultaneously with the latching of the auxiliary tool to the closure element, the primary latching mechanism is operated so that the closure element is unlatched from the bit body while remaining attached to the auxiliary tool.
The drill bit of the known system can be used for drilling operation, when the closure element is connected to the bit body. When it is desired to log the formation, drilling operation is stopped, and the logging tool string with the auxiliary tool at its lower end is lowered through the well string into the passageway. The secondary latching device is connected to the closure element, and, simultaneously, the primary latching device is operated so as to release the closure element from the bit body. Then, the logging tool can be lowered into the wellbore ahead of the well drilling bit from where logging can be performed. After logging has been completed, the logging tool string can be pulled back into the well string, so that the closure element is re-connected to the bit body and the auxiliary tool is simultaneously disconnected from the closure element.
Although the known system allows access to the open borehole ahead of the drill bit in the course of a drilling operation with a logging tool, it has the disadvantage that a special drill bit provided with passageway and removable closure element is required.